1. Field of the Invention
The invention relates to an image processing apparatus which is capable of carrying out interpolation processing of an image, for example, when a monochrome hand scanner skips reading.
2. Description of the Related Art
Traditionally, a scanner used in a facsimile and a copier is provided with image sensors such as CCDs or CMOSs placed in one line, thereby reading images on a document at individual lines sequentially while relatively moving the location of the document in a sub-scanning direction perpendicular to a main scanning direction, which is the direction of the lines, and forming digital data of a two-dimensional image. In contrast to a camera using a two-dimensional image sensor, a scanner uses a one-dimensional image sensor.
Methods for moving the relative location of an image sensor and a document are a flat bed scanner method of fixing a document and moving an image sensor and a sheet feed scanner method of fixing an image sensor and moving a document. Since both the methods need mechanisms for moving, a scanner becomes large and hard to move.
On the other hand, there is a scanner moved not mechanically but manually. This is called a hand scanner. Although a hand scanner is easily portable, it is difficult to keep a speed of manually moving the scanner constant. Therefore, there arises a case where the moving speed becomes higher than a speed at which the scanner is capable of reading and reading of a line is skipped.
When reading of a line is skipped, a read image is shrunk as compared with original one. In order to prevent shrinkage, there is a need to carry out interpolation processing. In the monochrome hand scanner, in order to save a memory capacity, an image is usually stored after being binarized. Therefore, for interpolation, a method of copying a previously read and binarized line to a skipped line is used.
When interpolation is carried out by the method of copying a binarized line, an image quality does not deteriorate extremely in the case of a character image for which simple binarization is used. However, when interpolation is carried out in the case of a halftone image by error diffusion or the like, an image quality deteriorates considerably because a thick cluster of black pixels of two lines is formed only at an interpolated part. The deterioration state of the image quality is apparent by comparing FIG. 4A and FIG. 4B. Therefore, interpolation is carried out only in a case where the subject is a character image, and not carried out in the case of a halftone image.